Field effect transistors are broadly used as unit electron elements of semiconductor memory integrated circuits, high-frequency signal amplification elements, liquid crystal driving elements and the like, and the transistors are electronic devices which have nowadays been put to practical use in the largest quantities.
Above all, with the remarkable development of the display devices in recent years, in not only liquid crystal display devices (LCDs) but also various display devices such as electroluminescence display devices (ELs) and field emission displays (FEDs), thin-film transistors (TFTs) are used in large quantities as switching elements for applying a driving voltage to display elements to drive the display devices.
Moreover, as the material of the transistors, a silicon semiconductor compound is most broadly used. Usually, for high-frequency amplification elements, elements for integrated circuits and the like which require a high speed operation, a silicon single crystal is used, and for the liquid crystal driving elements and the like, amorphous silicon is used for the requirement of a area enlargement of the display device.
Furthermore, in the case of a crystalline silicon-based thin film, a high temperature of, for example, 800° C. or more is required at its crystallization, and hence it is difficult to form the thin film on a glass substrate or an organic substrate. Therefore, the formation of the thin film is possible only on an expensive substrate such as a silicon wafer or quartz having a high heat resistance. In addition, for the production of the film, a large amount of energy and many steps are necessary.
Furthermore, in the crystalline silicon-based thin film, the element constitution of the TFT is usually limited to a top gate constitution, and hence it has been difficult to decrease the number of masks or the like, which disturbs the decrease of production costs.
On the other hand, an amorphous silicon semiconductor (amorphous silicon) which can be formed at a comparatively low temperature has a low switching speed as compared with a crystalline semiconductor. Therefore, when the amorphous silicon is used as the switching element for driving the display device, such an element cannot follow up the display of a dynamic image at a high speed on occasion.
Furthermore, when a semiconductor active layer is irradiated with visible light, the layer exerts conductivity, and a leakage current might be generated to cause malfunctions. Thus, properties as the switching element deteriorate on occasion. To solve this problem, a method of providing a light block layer which blocks the visible light is known, and as the light block layer, for example, a metal thin film is used. However, when the light block layer made of the metal thin film is provided, not only the steps increase, but also the layer has a floating potential. Therefore, the light block layer needs to be set to a ground level, and also in this case, a parasitic capacity is generated.
It is to be noted that at present, as the switching element for driving the display device, an element using a silicon-based semiconductor film has been a mainstream. This is because such a silicon thin film has various satisfactory performances such as a high stability, an excellent processability and a high switching speed. Moreover, such a silicon-based thin film is usually produced by a chemical vapor deposition (CVD) process.
Moreover, some of conventional thin-film transistors (TFTs) have an inversely staggered structure in which a gate electrode, a gate insulating layer, a semiconductor layer of hydrogenated amorphous silicon (a-Si:H) or the like, a source electrode and a drain electrode are laminated on a substrate of glass or the like. This kind of thin-film transistor is used as the driving element for not only an image sensor but also a flat panel display typified by an active matrix liquid crystal display in the field of a large area device. In these applications, even in the elements using conventional amorphous silicon, the speedup of an operation has been demanded with the advancement of functions.
Under such a situation, in recent years, a transparent semiconductor thin film made of a metal oxide has received attention as a thin film having an excellent stability as compared with the silicon-based semiconductor thin film (see Patent Documents 1 to 6).
In general, the electron mobility of oxide crystals increases as the overlap of the s-orbits of metal ions increases. The oxide crystals of Zn, In and Sn each having a large atomic number have a large electron mobility of 0.1 to 200 cm2/Vs. Furthermore, the oxide has ionic bonds between oxygen and metal ions, and hence it does not have any directionality of the chemical bonds. Hence, even in an amorphous state in which the direction of the bonds is nonuniform, the oxide can possess an electron mobility close to that of a crystalline state. In consequence, even if the metal oxide is amorphous, a transistor having a high field effect mobility can be produced, in contrast to the Si-based semiconductor. There have been investigated various semiconductor devices utilizing these advantages and made of the crystalline/amorphous metal oxide containing Zn, In and Sn, circuits using the semiconductor devices, and the like.
It is to be noted that the technology of a transparent conductive film obtained by adding a positive bivalent element to indium and (see Non-Patent Document 1), technologies concerning the crystal configuration of ITO (see Non-Patent Documents 2 and 3) and the like are investigated.
Moreover, various technologies are disclosed in which an amorphous transparent semiconductor film made of a metal oxide such as indium oxide, gallium oxide or zinc oxide is produced by, for example, a pulse laser deposition (PLD) process, and a thin-film transistor is driven (see Patent Documents 7 to 11).    Patent Document 1: JP-A-2006-165527    Patent Document 2: JP-A-11-505377    Patent Document 3: JP-A-60-198861    Patent Document 4: JP-A-2006-528843    Patent Document 5: JP-A-2006-502597    Patent Document 6: WO 2005/088726    Patent Document 7: JP-A-2003-86808    Patent Document 8: JP-A-2004-273614    Patent Document 9: JP-A-7-235219    Patent Document 10: JP-A-2006-165528    Patent Document 11: JP-A-2006-165532    Non-Patent Document 1: Thin Solid Films 445 (2003) 63 to 71    Non-Patent Document 2: Thin Solid Films 411 (2002) 17 to 22    Non-Patent Document 3: Journal of Applied Physics 100, 093706 (2006)